In a typical wireless, e.g. cellular, system, base stations (BSs) and mobile stations (MSs) communicate with each other over the air interface. Each BS controls a set of MSs, communicates with a set of MSs on the forward link (FL) from the BS to each MS, and listens to the communications from a set of MSs on the reverse link (RL) from each MS to the BS. For purposes of the discussion below, the flow of data may be bidirectional, and accordingly, each BS and each MS may function as both the transmitter and receiver of data.
The data flow over the air interface between the transmitter and receiver may take the form of encoder packets (EPs). The data to be transmitted is provided initially in the form of payload bits. An EP is a set of bits derived from the payload bits by applying a coding scheme and, typically, adding further bits related to control functions applicable to the link between the transmitter and the receiver. According to the coding scheme, a coding rule may be applied to add redundancy by mapping the payload bits to (typically a greater number of) coded bits.
An appropriate modulation scheme is then used to convert the EP bits into symbols appropriate for transmission over the air interface. Some examples of well-known modulation schemes are BPSK (binary phase shift keying), QPSK (quaternary phase shift keying), and QAM (quadrature amplitude modulation). Different modulation schemes may result in different numbers of bits being transmitted per symbol, i.e., per channel use. As a consequence, certain modulation schemes may work better than others for given channel conditions such as the signal to noise and interference ratio (SINR).
It is generally desirable to seek reliable reception of the EP, while also seeking insofar as possible to optimize the throughput, i.e. the number of bits carried per unit time between the transmitter and the receiver. An advantageous combination of these two performance measures may be sought through techniques such as any of the following, without limitation:
Coding, as mentioned above, to add redundancy leading to more reliable reception;
matching the modulation scheme to the characteristics, such as SINR, of the air interface link between the transmitter and receiver;
transmitting the EP with power sufficient to overcome the air interface channel impairments such as interference, since reception reliability will generally be at least roughly proportional to transmit power; and
repeating failed (e.g., erroneous) EP transmissions, for example upon notification by the receiver. In general, the chance of successful EP reception increases with the number of re-transmissions.
The control information included in an EP allows the receiver to do error detection, i.e., for the receiver to know whether or not the payload bits in the EP were received successfully. According to one well-known method of error detection, the transmitter computes a function value using the transmitted payload bits as the input. This function value is then carried within the EP, in the portion designated for carrying control information portion.
The receiver, on receipt of the EP, computes the value of the same function, using the decoded payload bits as the input. The receiver then compares the locally computed value with the value carried in the control portion of the EP. If the values match, the receiver knows that the payload bits have been correctly decoded.
In the discussion below, we will refer to the control information embedded in the EP for error detection as the error detection indicator (EDI). One particular example of an EDI is the well-known Error Detection Code (EDC).
Below, our discussion will mainly concern communication between the transmitter and receiver for supporting user application flows. That is, the payload bits carried by the EPs originate from user applications. By “user application,” we mean those software instructions and protocols that define, enable, and implement a packet-based communication service such as Internet-based video, VoIP, or any of various data and multimedia services. For example, a user may be accessing a multi-media website through the MS, in which case the communication between the MS and the BS would be to support the voice over IP (VoIP) and video application flows being downloaded from the website by the user.
An application flow (AF) may be characterized by its quality-of-service (QoS) requirements, which specify some parameters that must be met for that application flow to provide a user experience that is acceptable, for example according to qualitative criteria or according to specified quantitative criteria. For example, the QoS parameters for a flow may comprise a maximum amount by which flow packets may be delayed in transit from transmitter to receiver, and a minimum average throughput, i.e., number of flow bits sent from transmitter to receiver per unit time, that will correspond to a satisfactory user experience.
Since AF bits are carried from the transmitter to receiver via the EPs, the QoS associated with the AF will depend, at least in part, on the manner in which the AF bits are mapped to the EP payload bits. It will also depend, in part, on the coding scheme, modulation scheme, EP transmit power, and other parameters of the EPs that carry the AF. For example, if an AF has a tight delay requirement, the EP or EPs carrying that AF should be successfully received in very few transmission attempts. This, in turn, implies that codes with high redundancy, higher transmit power, or the like may be needed to assure dependable reception.
To map AF bits to the EP payload bits, at least some current networks follow the approach of mapping the bits from several distinct AFs into a common EP. The EP characteristics are then set to meet the most stringent of the several QoS requirements that apply to the respective AFs that the EP is intended to carry. For example, the transmit power for such an EP may be set based on the most stringent (i.e., lowest) delay requirement among the several AFs.
One drawback of such an approach is that by failing to consider the less stringent QoS requirements that apply to some AFs, it can waste transmitter resources. For example, setting the transmit power of the EP to the most stringent of the several QoS requirements will ensure that the AF subject to those requirements will meet them, but it also leads to the expenditure of resources for carrying other AFs with better performance than required. For example, it may lead to some AFs being carried with more than sufficient transmit power.